1.过滤器创建

了解过滤器常见配置含义

   <filter>
        <filter-name>filter名字</filter-name>
        <filter-class>对应到Java包名类名</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>filter名字</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

filter-mapping标签里面还有许多参数，可以参考学习博客
特别注意<filter-mapping>标签可以同一个过滤器写多个url-pattern
<filter-mapping>标签还可带参传入到java类中，使用<init-param><param-name><param-value>
在web.xml创建过滤器

      <filter>
        <filter-name>ElectronicFilter</filter-name>
        <filter-class>com.chinasoft.it.attendance.filter.ElectronicFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>ElectronicFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

2.Java类创建

1>.实现Filter 接口

2>.init方法可读取web.xml中<init-param>参数值

3>.重写doFilter方法，实现自己所要的功能。

4>.如果不过滤走filterChain.doFilter(request, response)方法;

public class FilterName implements Filter {

    protected String ip = "";
    protected List<String> listUrl;

    /**
     * @see Filter#destroy()
     */
    public void destroy() {
    }

    /**
     * @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)
     */
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain filterChain) throws IOException, ServletException {

        String ipRemoteAddr = request.getRemoteAddr();//读取发送请求IP地址
        //String reqUrl = ((HttpServletRequest) request).getRequestURI();

        if (!listUrl.contains(ipRemoteAddr)) {
            response.setCharacterEncoding("gb2312");
            PrintWriter out = response.getWriter();
            out.print("<p style='text-align:center; font-size:25'>IP限制！</p>");
        } else {
            filterChain.doFilter(request, response);
        }
    }

    /**
     * @see Filter#init(FilterConfig)
     */
    public void init(FilterConfig filterConfig) throws ServletException {
    
    }
}